Stringent requirements are imposed on binders for electrodeposition coatings. Not only is it necessary to prepare the electrodeposition bath and the ultimate electro-deposited coating without difficulty, but a very high profile of properties is also required in the binder such as throwing power during the electrodeposition and surface characteristics of the films produced, including corrosion resistance and stone-chip resistance in connection with good adhesion to the substrate being coated and to subsequent coats of paint. To achieve favorable dilution properties of the coatings in an electro-dipping plant, the binders should have a low viscosity. On the other hand, a high molecular weight is one of the preconditions for achieving good resistance properties in the final crosslinked films, which is usually associated with a high viscosity of the binder. Similar contradictory requirements exist in respect to the development of a high electrical film resistance during deposition in order to achieve a good throwing power, and the simultaneous desire for a high film thickness with excellent surface quality.
A compromise between an optimum in deposition characteristics and the properties of the crosslinked coating films is obtained in different ways by different types of modification of bisphenol A-epoxy resin-amine adducts or phenol novolak-epoxy resin-amine adducts as disclosed in the literature. Thus, EP-A2-0,355,654, corresponding to U.S. Pat. No. 4,992,516, describes a high molecular weight structural unit which contains oxirane functional groups and has a low glass transition temperature, with a polyhydroxy-diamine as a modifier, which enables the properties such as solubility properties, throwing power, layer build-up, and film flow to be optimized. It is also known that products carrying urethane or urea groups such as described in EP-A2-0,218,812, corresponding to U.S. Pat. No. 4,845,171, or in EP-B1-0,209,857, corresponding to U.S. Pat. No. 4,711,934, produce particularly good results in respect to their corrosion protective properties. The incorporation of these urethane or urea groups in general has the effect of increasing the glass transition temperatures of the binders. However, the consequence of this is a deterioration in the film flow and a reduction in the maximum layer thicknesses which can be achieved. To achieve high film layers and excellent surface qualities, modifications must be made to such binder systems to render the polymer structure flexible, which in most cases again causes a reduction in the resistance properties of the stoved films, especially at the edges of the workpieces, and a reduction in the adhesion of subsequent coats. The positive properties of these binders, that is to say their excellent corrosion resistance and stone-chip resistance in connection with the good adhesion to subsequent coats, can therefore only be partly utilized.
Many methods for plasticizing primers which can be deposited cathodically are known from the literature as disclosed, for example, in Austrian Pat. No. 381,115, corresponding to U.S. Pat. No. 4,659,800; U.S. Pat. No. 4,104,147, and EP-A2-074,634, corresponding to U.S. Pat. No. 4,419,467. However, such modifications, providing an increasing degree of plasticizing, result in further disadvantages such as a decrease in the wet film resistance and a deterioration in the throwing power of the coating.